Processes for growing carbon nanotubes using disordered carbon target

ABSTRACT

Processes for producing single-wall carbon nanotubes without catalysts are provided. The nanotubes are produced by vaporizing silicon carbide and carbon.

FIELD OF THE INVENTION

The present invention relates to processes for growing single-wallcarbon nanotubes in the absence of a catalyst.

BACKGROUND OF THE INVENTION

In the field of molecular nanoelectronics, few materials show as muchpromise as nanotubes, and in particular carbon nanotubes, which comprisehollow cylinders of graphite. Nanotubes can be incorporated intoelectronic devices such as diodes and transistors, depending on thenanotube's electrical characteristics. Nanotubes are unique for theirsize, shape, and physical properties. Structurally, a carbon-nanotuberesembles a hexagonal lattice of carbon rolled into a cylinder.

Besides exhibiting intriguing quantum behaviors at low temperature,carbon nanotubes exhibit the following important characteristics: ananotube can be either metallic or semiconductor depending on itschirality (i.e., conformational geometry). Metallic nanotubes can carryextremely large current densities. Semiconducting nanotubes can beelectrically switched on and off as field-effect transistors (FETs). Thetwo types may be covalently joined (sharing electrons). Thesecharacteristics point to nanotubes as excellent materials for makingnanometer-sized semiconductor circuits.

Nanotubes can be formed as single-wall carbon nanotube (SWNTs) ormulti-wall carbon nanotube (MWNTs). SWNTs can be produced, for example,by arc-discharge and laser ablation of a carbon target (U.S. Pat. No.6,183,714). Local growth of tubes on a surface can also be obtained bychemical vapor deposition (CVD). The growth of the nanotubes is madepossible by the presence of metallic particles, such as Co, Fe, and/orNi, acting as catalyst. The resultant carbon nanotubes comprisecontaminants, e.g., catalyst particles. For most potential nanotubeapplications, the use of clean nanotubes can be important, for example,where nanotubes are incorporated as an active part of electric devices.The presence of contaminating atoms and particles can alter theelectrical properties of the nanotubes. The metallic particles can beremoved; however the process of cleaning or purifying the nanotubes canbe complicated and can alter the quality of the nanotubes.

SWNTs have been identified as potential components of electronicdevices. The quality of nanotubes, e.g., their ability to act as asemiconductor, can be affected by contaminants. Therefore, a need existsfor a method of catalyst-free growth of single-wall carbon nanotubes.

U.S. Patent Application No. 2004/0035355 discloses a method for growingsingle-wall nanotubes comprising providing a silicon carbidesemiconductor wafer comprising a silicon face and a carbon face, andannealing the silicon carbide semiconductor wafer in a vacuum at atemperature of at least about 1,350° C. and a pressure of 10⁻9 Torrthereby inducing formation of single-wall carbon nanotubes on thesilicon face. The disclosed method utilizes relatively low pressures.

New and/or improved methods for making carbon nanotubes are desired.

SUMMARY OF THE INVENTION

One aspect of this invention is a process comprising:

-   -   a) providing a target comprising silicon carbide and carbon;    -   b) vaporizing the target in a catalyst-free environment in an        inert atmosphere at a pressure from about 10⁻³ Torr to about 000        Torr; and    -   c) forming a product comprising at least one single-wall carbon        nanotube.

Another aspect of the present invention is a single-wall carbon nanotubeproduced by a process comprising:

-   -   a) providing a target comprising silicon carbide and carbon;    -   b) vaporizing the target in a catalyst-free environment in an        inert atmosphere at a pressure from about 10⁻³ Torr to about 000        Torr; and    -   c) forming a product comprising at least one single-wall carbon        nanotube.

These and other aspects of the present invention will be apparent tothose skilled in the art, in view of the following specification and theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a transmission electron micrograph of agglomerates ofsingle wall carbon nanotubes produced by one embodiment of the presentprocess.

DETAILED DESCRIPTION OF THE INVENTION

All documents cited herein are expressly incorporated herein byreference in their entirety. Applicants also herein incorporate byreference the co-owned and concurrently filed application entitled“PROCESSES FOR GROWING CARBON NANOTUBES USING DISORDERED CARBON SOURCE”(Attorney Docket # CL 2627).

When an amount, concentration, or other value or parameter is given aseither a range, preferred range, or a list of upper preferable valuesand lower preferable values, this is to be understood as specificallydisclosing all ranges formed from any pair of any upper range limit orpreferred value and any lower range limit or preferred value, regardlessof whether ranges are separately disclosed. Where a range of numericalvalues is recited herein, unless otherwise stated, the range is intendedto include the endpoints thereof, and all integers and fractions withinthe range. It is not intended that the scope of the invention be limitedto the specific values recited when defining a range.

The present invention provides a process for growing single-wall carbonnanotubes (SWNTs) in the absence of a catalyst. The process includesproviding a target that is a mixture of silicon carbide and carbon,which comprises 50 weight percent silicon carbide or less, preferablyabout 45 weight percent or less, more preferably about 42 weight percentor less. In some embodiments, the amount of silicon carbide can be aslow as 1 weight percent. Preferably, the amount of silicon carbide inthe mixture is at least about 1 weight percent. The target comprisingthe mixture can be formed by, for example, forming a slurry of siliconcarbide and carbon powders in a volatile solvent, allowing the solventto evaporate, then compression molding the residual solid. Thecompression molded silicon carbide/carbon article can be optionallyheated, preferably in an inert atmosphere, to substantially removetraces of the solvent and harden the target. Other methods for preparingsuch a target are known to those skilled in the art.

Vaporization of the target can be carried out by laser ablation or othersuitable methods known to those skilled in the art, such as, forexample, rf induction heating and sputtering. The vaporization can becarried out at temperatures between about 100° C. and 1500° C. andpressures of vacuum (e.g., about 10⁻³ Torr) to above atmosphericpressure.

In some preferred embodiments, the vaporization is carried out at atemperature from about 1000° C. to about 1200° C. Preferably, thevaporization is carried out in the presence of a non-oxidizing gas, suchas argon, neon, helium, nitrogen or mixtures thereof. By“non-oxidizing”, as used herein, is meant an atmosphere in which oxygencontent is minimized. Minimization of oxygen in the atmosphere duringnanotube production is desirable because oxygen can oxidize the carbon,thereby reducing the production of the desired nanotubes. However, thetotal absence of oxygen is not required. Thus, in one illustrativeembodiment, the target material is mixed, pressed and heated in an inertatmosphere at 1150° C. to harden the target before it is placed into alaser ablation system, wherein the oxygen content is minimized.Generally it is preferred that the atmosphere comprise no more thanabout 100 ppm oxygen, preferably about 50 ppm or less, more preferablyabout 25 ppm or less. Carbon nanotubes can desirably be formed in thepresence of a non-oxidizing gas, such as argon, neon, helium, nitrogenor mixtures thereof. Commercially available tanks of gases, such as99.9% pure argon, are suitable for the process of forming carbonnanotubes.

While the selection of the gas under which vaporization is carried outis not critical, the nature of the gas can affect the amount ofnanotubes produced. While it is not intended that the invention be boundby any particular theory, it is believed that the thermal conductivityof the gas can affect the formation of nanotubes. For example, the useof helium may result in the formation of fewer nanotubes than would theuse of nitrogen, because the higher degree of cooling expected to occurwith helium can result in a cooler, and therefore less active, growthzone.

In one embodiment of this invention, a SWNT-containing product producedusing the processes disclosed herein can serve as a target for one ormore additional cycles of vaporization and SWNT-formation.

The process can further comprise an annealing step. Annealing does notrequire substantial additional processing, and can be accomplished byallowing the newly formed nanotubes to remain undisturbed and coolfollowing ablation. The annealing can be performed in an ultra-highvacuum (UHV) (e.g., at a pressure less than about 10⁻⁹ Torr), or athigher pressures, even above atmospheric pressure (760 torr). Generally,a pressure of about 500 torr is suitable.

It is generally desirable to grow the tubes at pressures of at least 1millitor, and preferably at 500 Torr or above. In some preferredembodiments, the pressure is about 1000 Torr. It is generally notdesirable that the pressure be greater than about 1000 Torr. Although areduction in pressure below about 500 Torr has not been observed toundesirably affect the rate of growth of nanotubes, pressures of about500 Torr or greater are often practical. In embodiments, the pressure isfrom about 300 Torr to about 600 Torr. The nanotubes that are formed arepredominately SWNTs as shown in the Figure. The SWNTs can be very longand have a good crystalline quality. “Good crystalline quality” meanssubstantially free of observable defects under transmission electronmicroscopy.

All of the compositions and processes disclosed and claimed herein canbe made and executed without undue experimentation in light of thepresent disclosure. While the compositions and processes have beendescribed in terms of preferred embodiments, it will be apparent tothose of skill in the art that variations can be applied to theprocesses and methods and in the steps or in the sequence of steps ofthe processes described herein without departing from the concept,spirit, and scope of the invention. All substitutions and modificationsapparent to those skilled in the art are deemed to be within the spirit,scope, and concept of the invention as defined by the appended claims.

EXAMPLE

The present invention is further defined in the following Example. Itshould be understood that this Example, while indicating a preferredembodiment of the invention, is given by way of illustration only. Fromthe above discussion and this Example, one skilled in the art canascertain the preferred features of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications to adapt it to various uses and conditions. ThisExample shows the production of single wall nanotubes by vaporizingsilicon carbide with no added catalyst.

A silicon carbide target was made by mixing 36.1 grams (g) of siliconcarbide powder (Third Millennium Technologies, Inc., Knoxville, Tenn.)with 51.5 g of Dylon®graphite cement (Dylon Industries, Inc., Cleveland,Ohio) in 80 ml of methanol. The methanol was allowed to evaporateovernight. The remaining solid was broken into small pieces in a mortarand pestle and compression molded at 130° C. for 1 hour. The moldedarticle was then baked at 1150° C. for 10 hours in flowing Ar, and theninserted into a furnace at 1100° C. The target was ablated with Nd—Yaglasers running at 30 Hz with a pulse width of 10 nanoseconds. Thepressure in the furnace was maintained at 500 torr. The target wasrotated during ablation to achieve even ablation, and 1.12 g of productwere collected after 1 h of run time. The micrographs in FIG. 1 show thepresence of single wall carbon nanotubes.

1. A process comprising: a) providing a target comprising siliconcarbide and carbon; b) vaporizing the target in a catalyst-freeenvironment in an inert atmosphere at a pressure from about 10⁻³ Torr to1000 Torr); and c) forming a product comprising at least one single-wallcarbon nanotube.
 2. The process of claim 1, wherein the vaporizationstep is carried out by laser ablation.
 3. The process of claim 2,wherein the laser ablation is performed at a temperature from about 100°C. to about 1500° C.
 4. The process of claim 2, wherein the laserablation is performed at a temperature from about 1000° C. to about1200° C.
 5. The process of claim 1, wherein the pressure is about 500Torr or greater.
 6. The process of claim 1, further comprising anannealing step after the formation of the at least one single-wallcarbon nanotube.
 7. The process of claim 1, wherein the vaporization iscarried out in the presence of an inert gas selected from argon, neon,helium, nitrogen and mixtures thereof.
 8. A single-wall carbon nanotubeproduced by the process of claim 1.